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Physica E 41 (2009) 1197–1200

Contents lists available at ScienceDirect

Physica E

1386-94

doi:10.1

� Corr

E-m

journal homepage: www.elsevier.com/locate/physe

Massive production of nanoparticles via mist reaction

Ran Liu �, Lei Liu, Jing Liu

Department of Biomedical Engineering, School of Medicine, Tsinghua University, Beijing 100084, China

a r t i c l e i n f o

Article history:

Received 18 August 2008

Received in revised form

24 December 2008

Accepted 27 January 2009Available online 4 February 2009

PACS:

81.07.�b

81.16.�c

82.30.�b

Keywords:

Mist reaction

Nanoparticle

Mass production

Microdroplet

77/$ - see front matter & 2009 Elsevier B.V. A

016/j.physe.2009.01.012

esponding author. Tel.: +86 10 6278 8963.

ail address: liuran@tsinghua.edu.cn (R. Liu).

a b s t r a c t

A novel conceptual nanoparticle fabrication method is proposed in this paper. It can be easily

implemented for the preparation of micro or nanoparticles through a reaction between mists with

different specific chemical compounds produced by ultrasonic atomization technology. Ultrasonic

atomization is an established technology that easily atomizes liquid to produce very small droplets—in

the orders of tens to hundreds of micrometers. The results reveal that metal oxide nanoparticles, such as

iron oxide can be massively produced via reactions between metal chlorides and sodium carbonate in

an experimental set-up based on physical and chemical principles. The density of the nanoparticle

distribution is also investigated and determined to be dependent on the amount of mist reacted and the

collection time. Moreover, since the vibrational frequency of ultrasound can be adjusted, we can control

the size of micro-droplets of reactants, hence producing particles of different dimensions. Given that the

double mist reaction method is easily controllable, environmentally friendly and extremely low in cost,

it can potentially become a significant method for making micro/nano particles in the newly emerging

field of nanofabrication and integration.

& 2009 Elsevier B.V. All rights reserved.

1. Introduction

When particle dimensions are reduced to the order of severalnanometers, their physical and chemical properties deviatesignificantly from the bulk properties of such materials. Becauseof this, there is abundant potential for their use in futuretechnologies including electronic and optoelectronic, mechanical,chemical, cosmetic, medical, drug, and food technologies [1].There has been an increasing interest in the development of novelnanoparticle and microparticle technologies for drug delivery,imaging, bioanalysis, photonics, and optoelectronic applications[2]. However, due to the expensive equipment and demandingreaction conditions such as high temperature and pressure inextremely small spaces, nanoparticle and microparticle fabrica-tion processes are generally limited. A big challenge facing thescientific society today is to overcome this barrier and build aconnection between macroscopic manipulation/observation andsmall-scale fabrication [3]. Aiming to establish a highly flexible,straight forward—and inexpensive way to fabricate nanoparticles,we proposed here to utilize chemical reactions between mistdroplets consisting of different specific chemical compounds,produced by ultrasonic atomization technology. Ultrasonic ato-mization is an established technology that easily atomizes liquidto produce very small droplets—in the orders of tens to hundreds

ll rights reserved.

of micrometers. Due to its simplicity and low-cost, this method isexpected to be used in fabrication of nanoparticles. In this paper,the basic principle and typical applications for the new methodwas illustrated.

2. Principle of double mist reaction method

The principle of the double mist microfabrication method isbased on both physics and chemistry (Fig. 1). The main aim is tomake two solutions chemically react in their physical states ofatomization. One advantage of this method is that through theprocess, the products are miniaturized to tiny particles or a finespray in a micro- or even nano-scale, which will be much smallerthan those produced by reacting two solutions directly. It is alsowell known that through ultrasonic atomization, liquid can bevery easily transformed into very small droplets in the order oftens to hundreds of micrometers. Before fabrication, one needs toprepare two kinds of soluble reagents A and B, which could reacttogether and synthesize into two new compounds C and D. Cshould be insoluble and behaves as an etchant operating on thesubstrate, while D should be soluble. This reaction process can beexpressed as A+B-C+D.

Based on the previous experimental research result of Lang[4,5], we can estimate the diameter (d) of the microdroplet, afterultrasonic atomization, using the formula below

d ¼ 0:34ð8ps=rf 2Þ1=3

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Fig. 1. (a) Schematic representation of using double mist reaction for producing

nanoparticles. The process includes the atomization step by ultrasound technol-

ogy, chemical reaction between the two kinds of droplets, collection of product

droplets, and cleaning to get rid of unwanted solvent. (b) The process illustrated in

terms of droplets: the red and green droplets represent the reactant droplets,

respectively, while the blue and orange droplets represent the product droplets, in

which the blue ones are the desired nanoparticles. Note that the size of blue

droplets will be much smaller than the size of the reactant droplets. (For

interpretation of the references to color in this figure legend, the reader is referred

to the web version of this article.)

Fig. 2. The phenomenon of air evaporation will follow the droplets, when they

leave the liquid. When the two droplets collide or the three droplets meet, the

results are same. The area of reaction is where the blue and the green droplets

overlap. (For interpretation of the references to color in this figure legend, the

reader is referred to the web version of this article.)

Fig. 3. Sketch of the experimental system.

R. Liu et al. / Physica E 41 (2009) 1197–12001198

where f is the vibrational frequency of ultrasound, s is the surfacetension of liquid, and r is the density of liquid. In addition, rcorrelates with the molar concentration of solution (c) and thetemperature (T). For a solution at a given concentration andtemperature, d depends on the vibrational frequency f. Therefore,the sizes of the droplets of solutions A and B can be changed byadjusting frequency.

Once the droplets leave the liquid, the phenomenon of airevaporation will follow (Fig. 2). When the liquid–gas interface is aflat surface, the amount of evaporation is related to resistance,differential concentration, area and it can be expressed in terms of

a differential equation, as shown below:

dQ

dt¼ AptDc

where Q is the amount of passing mass in t seconds through asurface area A, Dc is the differential concentration between thetwo phases and pt is the penetrant coefficient. 1/pt ¼ Rt is thepenetrant resistance (s m�1), which includes the diffusion resis-tance between liquid phase molecules, the diffusion resistancebetween gas phase molecules and the resistance caused by theinterfacial molecules, which are undergoing state changes [6,7].For a droplet, Q varies inversely with the square of diameter (d2).When the diameter is reduced by a hundred times, the evapora-tion rate will increase by ten thousand times [8–10]. For example,a water droplet with the diameter of 10mm can be vaporizedcompletely in 60 ms at 18 1C. Therefore, when droplets of A and Bmeet and react, their sizes are much smaller than that justatomized, which is about 3–5mm at 1.7–2.4 MHz. Two smalldroplets of the solutions A and B collide in the reaction chamberand their reaction area is given by the contact surface of droplets.As the chemical reaction proceeds, the mass of the solutions A andB is reduced. Through ionic diffusion, droplets will movecontinuously to the reacting area. After vaporization and reactionprocesses, the droplet size of products C and D will be furtherreduced to a nano-scale. Since solution D is soluble, it can beremoved by washing after drying in a particle collector. Purenanoparticles of the product C would then be obtained.

3. Demonstration experiments

To demonstrate the capability of the double mist reaction formassive production of nanoparticles, a series of conceptualexperiments were carried out in this paper. We designed andbuilt an atomization reaction system, consisting of two ultrasonicatomizers, a reaction chamber, many collectors, and pipelineconnection parts (Fig. 3). To produce metal oxide nanoparticles bythis macroscopic manipulation system at room temperature andpressure, we used solutions of ferric chloride and sodiumcarbonate, as the reactant solutions A and B, respectively, whoseconcentrations are carefully prepared as 0.1 mol/l. Firstly, the tworeagents of ferric chloride and sodium carbonate were, respec-tively, atomized and the two kinds of droplets were blown into thereaction chamber by a fan. Then, they collided continuously andbegan to gradually react in the chamber. After a few seconds, thesmaller droplets of ferric oxide and sodium chloride (C and D)were synthesized and dropped onto the particle collector. In thissystem, we designed a very narrow outlet to ensure the complete

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R. Liu et al. / Physica E 41 (2009) 1197–1200 1199

reaction in the chamber. In this way, nanoparticles of ferric oxidewere left on the wafer after the droplets were dried and theunwanted solvent was drained away. In order to confirm theparticle quality, X-ray diffraction was performed on wafer withparticles before and after unwanted solvent was cleaned away.X-ray diffraction measurements were carried out using a RigakuH-12 diffractometer in ‘‘2y’’ mode. Firstly, the diffractogram of thesample showed four peaks, confirming the presence of sodiumchloride (Fig. 4a), and these four peaks disappeared after thesample was washed and dried, but the peaks of ferric oxide werestill present (Fig. 4b). Therefore, the unwanted solvent can beremoved in this way. EDS of the wafer also expressed that theferrum and oxygen were the main elements and other elements’contents were very low, besides the elements of wafer (Fig. 5c).

The scanning electron microscope (SEM) images of ferricchloride nanoparticles fabricated by following the above experi-mental procedures are shown in Fig. 4. As indicated by themeasuring scale, the photograph (Fig. 5a) is for a particle withaverage width of 80 nm. Based on the factor analysis above, wecan produce droplets and hence nanoparticles of different sizes bycontrolling the vibrational frequency of ultrasound, the density ofliquid, and the time of evaporation. Moreover, the time ofcollection also affects the formation of nanoparticles. Longercollection time would increase the accumulation of nanoparticlesand induce non-uniform particle distribution (Fig. 5b). Clearly,other metal oxide nanoparticles such as zinc oxide could also beenmade using this method. However, a complete investigation on

Fig. 4. (a) XRD diffractogram for the nanoparticles including sodium chloride via mist

was washed and dried.

this interesting issue is beyond the scope of the present paper.Tremendous works are worth of pursuing in the near future.Moreover, in order to utilize the unique physical and chemicalproperties of nanoparticles, clustering should be avoided. Cluster-ing often occurs due to long reaction time. Thus, if we accuratelycontrol the collection time, velocity of gas flow and contact areaamong gas particles, clustering can be prevented. In general,collection time within half a minute is appropriate.

4. Discussion and conclusion

Herein, we demonstrate that the new double mist reaction is auseful technology for massive production of homogeneous nano-particles at a standard condition of temperature and pressure.

Through the analysis of the experimental results, the sizes ofnanoparticles are about 50–100mm and very homogeneous, as thevibrational frequency of ultrasound remains unchanged. However,if the collection time is lengthened, the distribution of nanopar-ticles will become dense and the clustering of nanoparticles willalso happen. In this way, nanoparticles with different distributiondensities, fabricated by this method, can be applied according todifferent needs. Moreover, since the vibrational frequency ofultrasound can be adjusted, we can control the size of micro-droplets of reactants, hence producing product particles ofdifferent dimensions. A series of interesting nanomaterials couldbe fabricated by this double mist reaction method, which also

reaction; (b) XRD diffractogram for the ferric oxide nanoparticles after the sample

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Fig. 5. (a) SEM picture of Fe3O2 nanoparticles after a short collection time; (b) SEM picture of Fe3O2 nanoparticles after a long collection time, (c) EDS of the wafer expressed

that the ferrum and oxygen were the main elements and the other elements’ contents were very low besides the elements of wafer.

R. Liu et al. / Physica E 41 (2009) 1197–12001200

builds up a bridge between the macroscopic manipulation/observation and the small-scale fabrication. Being easily con-trollable, environmentally friendly and extremely low in cost, thedouble mist reaction method is suitable of producing micro/nanoparticles, which is expected to be a significant technicalmethod for making micro/nanoparticles in the near future.

Acknowledgments

This research is partially supported by the National NaturalScience Foundation of China under the Grant 60701001, the WuShunde Medical Research Foundation and the Tsinghua-Yue-YuanMedical Sciences Found.

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